Technical Question

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flyguy

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This image was taken with a brand new Nikon D780 and 24-70 f/2.8 Nikkor non VR; the shot was taken at f/5.6, at close range, less than six feet to the subject which is between 6 and 7 inches in diameter, and hand held
A78_0160.jpg
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. The question is....why the apparent shallow depth of field on this shot? I would think at f/5.6 the entire fungus would have been in focus better than what I got?
 
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In Photopills F5.6 from 5 feet and 24mm on a D780, the depth of field would have been 5 feet - 18 inches in front of and 3 feet behind the point of focus which would have been ample. At 70mm you would have had 6 inches total DOF.
 
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Metadata on the image is as follows: f/5.6, 1/100, ISO-100, exposure bias 0 step, metering mode "pattern", aka "matrix", exposure program "Manual". Again, estimated distance to subject about 5 feet, and estimated depth of the fungus about 5 or 6 inches. Next time I will take the shot with my 70-200; I know exactly where this subject is at.
 
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Pistnbroke said:
The closer you get the less depth of field so f5.6 was not enough in this case. If you use the depth of field calculator below you will see the DOF is 6 inches... F11 would have given you 1 ft. Also remember if you focus on the front of the object then you loose half the depth of field.
Depth of Field (DoF) calculator | PhotoPills
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Not 1/2, 1/3. Hyperfocal is 1/3 in front, 2/3 behind.
 
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Warren D said:
Not 1/2, 1/3. Hyperfocal is 1/3 in front, 2/3 behind.
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But this wasn't shot at hyperfocal distance, which for a 48mm focal length is nearly 50 feet away.

Edit: this was staring me in the face and yet I didn't see it originally: hyperfocal distance is where you get everything from the focus point to infinity in acceptable focus, so by definition that is also not 1/3 in front and 2/3 behind. In fact, after twiddling with some DOF calculators, you only get 1/3 & 2/3 at a focus distance of 15 feet for this focal length. Progressively closer approaches 50/50, and progressively further away approaches 1% / 99% until you hit hyperfocal distance.
 
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flyguy said:
Focus point was the leading edge of the fungus.
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That choice cut your usable DoF roughly in half as all the DoF in front of the plane of focus wasn't helping you. Focusing on portions of the fungus a bit deeper into the frame would have helped you keep more of the growth in focus as would stopping down a bit further as in f/8 or a higher.
 
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flyguy said:
Metadata on the image is as follows: f/5.6, 1/100, ISO-100, exposure bias 0 step, metering mode "pattern", aka "matrix", exposure program "Manual". Again, estimated distance to subject about 5 feet, and estimated depth of the fungus about 5 or 6 inches. Next time I will take the shot with my 70-200; I know exactly where this subject is at.
Click to expand...

The discussion in this thread is informative. The image posted by the OP is cropped from the 2:3 ratio of the full frame sensor. Assuming that it was cropped in the horizontal direction and leaving the vertical dimension intact, I calculate that the fungus measuring 7 inches in the horizontal plane would occupy about 0.88 inches on the sensor, giving a magnification of about 0.125 or about 1:8. This may be off, but will be good enough for the discussion.

The usual depth of field calculations and tables for a full frame sensor assume a circle of confusion of 30 microns. This is for an 8 by 10 inch print viewed at 10 inches for an observer with manufacturer assumed visual acuity. This is from the PhotoPills DOF app. For 20/20 vision, the COF shrinks to 9 microns according to the app. Standard DOF assumptions are very lenient, and DOF with critical viewing is considerably smaller.

Rik Littlefield, the author of Zerene Stacker, has another way to calculate DOF. He uses magnification which eliminates focal length as a factor, since for a given magnification DOF is independent of the focal length. If the OP had framed the image the same with a 200 mm lens at f/5.6, the DOF would be the same as with the zoom lens at 48 mm. However, the perspective would be different.

Look at Table 2-B of Rik's article. For a magnification of 0.125 the DOF would be between 3.5 and 6.9 centimeters at f/5.6 (1.4 and 1.7 inches respectively). Using the equation shown below the table, this works out to be 4.4 cm. This DOF could be achieved only with stacking. For manual stacking one could measure the depth of the fungus (about 6 inches according to the OP) and focus from front to back stepping in somewhat smaller increments to allow some overlap until the far end of the fungus is reached.

This is pretty involved, but with the "focus shift" capability of the D780 this could be automated and accomplished in less than a minute. Steve's Deep Focus Techniques supplement to his autofocus for Nikon e-book has an excellent discussion of focus shift with Nikons as well as explanations of hyperfocal focusing and other DOF topics discussed in this thread.

This is a rather long post, but I hope it will help with using DOF in practical work.

Cheers,

Bill
 
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